In recent years, a so-called open joint method has been adopted as a watertight method for precast concrete curtain walls. This open joint method is capable of smoothly discharging rainwater driven in through a joint by having an opening with a gap of an appropriate width in the joint so as to have an isobaric space therein. This isobaric space has no pressure differential from the outside. Heretofore, as shown in FIG. 11, a method has been adopted which uses a double seal method that is constructed so as to prevent water from penetrating into the joint by filling up the front portion of a joint S with a gun grade sealant (1) on to a backing material (2) and mounting a preformed elastic sealant (3) at the back of the joint to form a watertight layer. However, if the life of the sealant is relatively shorter than that of the precast concrete curtain wall or the like and defects occur in the seal (for instance, interlaminar deviations or other problems), water driven in through the joint may created a pressure differential within the joint and thereby cause water leakage.
As shown in FIG. 12, the open joint method exhibits a watertight effect by an isobaric space BS. The isobaric space, however, is open to the exterior so that it may become a passage through which fire flames could enter. In this case, only a wind barrier (5) mounted at the back of the joint is not satisfactory for fire-resistant performance so that the exterior walls need to be thick enough for fire resistance. In particular, the vertical joint is not provided with a flashing (6) as is used in a horizontal joint, as shown in FIG. 13. Thus, the improvements for the vertical joint in fire-resistant performance are restricted. Of course, as shown in FIG. 11, such fire-resistant performance can be provided on the vertical joint if a fire-resistant material (4) would be filled in the joint. It is to be understood, however, that this loses the meaning of the open joint method which utilizes the isobaric space.